Development and performance assessment of polyurethane modified bitumen utilizing castor oil as a sustainable feedstock

Abstract

The development of novel eco-friendly polyurethane bitumen modifiers utilizing sustainable bio-based resources was currently a research hotspot. Castor oil (CO), with its unique chemical structure (containing hydroxyl groups) and bio-renewable advantages, represented an ideal choice for preparing bio-based PU. This study aimed to develop a high-performance, environmentally friendly renewable castor oil-based polyurethane modified bitumen (CO-PUMB), using petroleum-based polyurethane modified bitumen (P-PUMB) as a comparison, to provide insights for advancing the greening of road construction materials. First, the PU composition (isocyanate index R = 1, hard segment content C_h = 20 %) was designed and optimized, and determine the optimal hydroxyl substitution ratio for CO-substituted petroleum-derived polytetrahydrofuran (PTMG). Subsequently, the preparation process (90 °C, 400 rpm, 2 min) and the curing process (100 °C, 2 h) were established. Then, five types of CO-PUMB and P-PUMB with varying PU contents were prepared and subjected to microscopic morphology observation, physical performance testing, and rheological testing. Fluorescence microscopy observation indicated that, benefiting from the trihydroxy structure and low molecular weight of castor oil (CO), CO-PU formed an earlier, more extensive, and more efficient chemically cross-linked network than P-PU at a lower PU content (≥40 %). In contrast, P-PU relied primarily on physical entanglement and required a higher PU content (60 %) to achieve cross-linking. Physical performance and rheological tests demonstrated that CO-PUMB, leveraging its dense chemical cross-linked network, exhibited significantly superior high-temperature stability, fatigue life, elastic recovery, and resistance to permanent deformation compared to P-PUMB. Additionally, increasing the CO/P-PU content enhanced the overall performance of CO/P-PUMB across the entire frequency (temperature) range. Notably, the performance improvement in CO-PUMB within the high-frequency (low-temperature) region was dependent on an effective crosslinking network, while that in P-PUMB relied on the high motional freedom generated by PTMG molecules. The experiments demonstrated that CO-PUMB incorporating optimized CO substitution (≥40 % CO-PU) represented an environmentally friendly, high-performance bitumen material with excellent high and low temperature performance and anti-fatigue capability.